How do I work with grayscale?
The most elegant solution is using color management and a custom matrix to convert the image's color information to a grayscale.

You can easily and non-destructively represent your scene in grayscale without the need of changing materials or affecting the overall quality of the render.

User @troy_s has created a LUT pack and OpenColorIO (OCIO) config that greatly enhances blender's display transform.

Included in the pack there are a couple of matrices that can be used to set grayscale looks. 


  
  Greyscale on Desaturation This is a greyscale preview of the desaturated look helpful for evaluating contrast and other details in some contexts.
  Greyscale This is a greyscale preview without the desaturation look applied.
  




LUTs and OCIO configuration can be downloaded at this Github link along with the instructions on how to install and use.

For other related topics read this link